Demanding medical procedures often require the subject or the patient to be unaware of the procedure. In such surgeries, the patient is anesthetized to achieve a total loss of consciousness and sensation so that the surgery can be conducted without the patient being aware of it. Anesthesia is achieved by anesthetic drugs which are well known for having a potent effect on the brain cell system. The drug can be given to the patient in one bolus injection or it can be given continuously through a motor driven syringe pump or through an electronic anesthetic gas vaporizer via an anesthetic mask or endotracheal tube. When administration of the drug is by inhalation, the anesthetic agent comprises a volatile liquid that is vaporized in a vaporizer. The vaporized anesthetic agent is entrained in the breathing gases for the patient.
Usually, the effect of an anesthetic drug is short-term e.g. a few minutes because it is important that the patient goes under anesthesia quickly and awakes shortly after the surgery. When the drug is given constantly to the patient the duration of anesthesia can be controlled by adding the anesthetic drug as long as needed.
The extent to which the patient is anesthetized is often termed “adequacy of anesthesia”. As the magnitude of anesthetization increases, the anesthetized patient typically fails to respond to spoken commands, he loses reflexes and undergoes depression of vital signs, and the like. Anesthesia can be considered to comprise of three attributes: unconsciousness i.e. hypnosis, loss of pain sensation i.e. analgesia and muscular relaxation. Sufficient muscular relaxation is important for the surgeon for
example to be able to access the target part of the body without unnecessarily cutting through the muscles. When the neuromuscular junction is blocked by giving such drugs to the patient, the muscles do not respond to movement commands from the motor nerves.
Monitoring the depth of anesthesia is important when assuring that the patient remains unconscious and without sensations during the whole time of the surgery. Typically, the depth of anesthesia is wanted on a certain level depending on the procedure and each phase of the procedure. For example, at the beginning of the operation, a very heavy level of anesthetization is needed, whereas at the end when the surgeon is left with stitching up the wound, the anesthesia level can be brought up to a state of less deep sleep. Therefore, to be able to adjust the depth of anesthesia, its existing level in the patient is sensed and used to control the anesthetic drug administration to the patient. This manner of controlling the given amount of drug to achieve and maintain a desired level in the patient is called closed loop, or feedback, administration of anesthetic drug.
The importance of controlling the adequate anesthesia level is essential in common clinical practice and it is therefore a subject of constant attention and research. Inadequate anesthesia may result in the patient knowing or sensing stages of the procedure and as a consequence, obtain traumatic experiences. Excessively deep anesthetization on the other hand prolongs the recovery from the anesthetization and causes nausea and queasiness. In addition, most anesthetic drugs are expensive and their excessive use should, therefore, be avoided.
Another reason for the rising interest to monitor depth of anesthesia is its difficulty. Anesthetic agents manipulate the state of the patient's brain and these alterations of the brain are not easy to detect. However, it has long been known that the neurological activity of the brain is reflected in biopotentials available on the surface of the brain and on the scalp. Thus, efforts to quantify the extent of anesthesia induced hypnosis have turned to a study of these biopotentials. The biopotential electrical signals recorded on the scalp comprise an electroencephalogram (EEG). Several different methods for determining the hypnotic state based on the measured EEG signal have been introduced in literature. Among these are e.g. deriving from the EEG signal a bispectral index (BIS) that correlates behavioral assessments of sedation and hypnosis. Another recently obtained signal derived from the EEG is the entropy signal which describes the irregularity of the EEG and FEMG (frontalis electromyography) signals.
Entropy, as a physical concept, describes the state of disorder of a physical system. When used in signal analysis, entropy addresses and describes the complexity, unpredictability, or randomness characteristics of a signal. In a simple example, a signal in which sequential values are alternately of one fixed magnitude and then of another fixed magnitude has an entropy of zero, i.e. the signal is predictable. A signal in which sequential values are generated by a random number generator has greater complexity and a higher entropy.
Applying the concept of entropy to the brain, the premise is that when a person is awake, the mind is full of activity and hence the state of the brain is more complex, and noise like. Since EEG signals reflect the underlying state of brain activity, this is reflected in relatively more randomness and complexity in the EEG signal data and therefore, higher entropy. As a person falls asleep or is anesthetized, the brain function begins to lessen and becomes more orderly and regular and has, therefore, lower entropy. Similarly to EEG, the frontalis electromyography (FEMG) signal quiets down as the deeper parts of the brain are increasingly saturated with anesthetics.
Each patient has a unique way of responding to the drug. The amount of drug that totally knocks out a smaller patient, might not cause a heavier patient anything more than a funny feeling. Also, drug abusers might have developed a special resistance to drugs. The unique process by which a drug, such as an anesthetic drug, takes its effect in the body, has two important aspects: pharmacokinetics and pharmacodynamics. Pharmacokinetics deals with the effect of the distribution of the drug, such as the body's absorption, transport or diffusion, metabolism, and excretion of the drug. Pharmacodynamics describes how the drug is affecting a particular organ where the drug is supposed to have its effect.
Traditionally, determining the level of anesthesia has been dependent on the doctor's experience and professional skill. Adequacy of anesthesia is routinely assessed by subjectively observing the patient's clinical signs, such as heart rate, blood pressure, lacrimation, sweating and movement. However, these indices give an indirect indication of the actual state of consciousness. This method for determining the adequacy of anesthesia is neither suitable if the procedure requires heavy muscle relaxants e.g. abdominal surgery. In these kind of operations observing the patient's responses to external stimuli is completely inadequate to determine the level of anesthesia. In the worst case scenario, the patient is given strong muscle relaxants and is, therefore, immobile but is actually awake and aware of the surgery the whole time whereas the medical staff believes him to be heavily anesthetized. To prevent such traumas, the adequate level of anesthesia becomes even more important.
Since the anesthesia level is dependent on the amount of drug given to the patient, the depth of anesthesia can be controlled by adjusting the infusion rate of the drug. Adjusting the depth of anesthesia is based on the anesthesiologist's professional skill when he/she estimates how much a certain change in the infusion rate of the anesthetic drug affects the depth of anesthesia. Traditionally, the anesthesiologist adjusts the level of anesthesia e.g. by reducing the infusion rate of the drug to achieve a less deep state of sleep, and if the anesthesia level is too light (closer to the awake state), the anesthesiologist increases the infusion rate until the desired anesthesia level is achieved. Analogously, if anesthesia should be deepened, the anesthesiologist increases the infusion rate until the desired anesthesia level is reached.
The problem when adjusting the anesthesia level is that the only way to find out the effect of an increase or a decrease in the infusion rate is to test it and observe the response the infusion rate change has on the anesthesia level. Due to the unique way each patient responds to a drug and due to the differences in how the drug accumulates in the patient's body over time, it is difficult to know how much a change in the infusion rate actually changes the anesthesia level at each moment.